New Mexico Mineral Symposium — Abstracts

Mineralogical and geochemical studies of cave formations (speleothems), bedrock, and cave-filling mud can help us better understand the depositional processes and environments in caves. Samples were sparingly and carefully collected in collaboration with cave owners, members of the Cave Research Foundation (CRF), museum curators, and other USGS scientists. X-ray diffraction (XRD), scanning electron microscopy, electron microprobe analysis, bulk chemical analysis, optical microscopy and petrography, and cathodoluininescence microscopy have been used to examine samples.

Silent Splendor, a chamber within Cave of the Winds (a phreatic cave system developed in Ordovician to Mississippian limestone), Manitou Springs, Colorado, was completely sealed by mud-choked passages until it was first entered by cavers in 1984. This pristine 65-m-long room is noted for a great variety of speleothems (see Hill and Seanor, CRF 1986 Annual Report, p. 21¬24; Modreski and others, 1987, GSA Abstracts with Programs, v. 19, no. 5, p. 322), particularly its exquisite clusters of beaded helictites up to 30 cm long. Minerals in Silent Splendor include calcite, aragonite, hydromagnesite, gypsum, and "limonite".

Porcupine Cave, noted for a wide variety of Pleistocene fossil vertebrate remains, is developed in the Ordovician Manitou Dolomite at the southwestern edge of South Park, Colorado. In addition to typical carbonate speleothems, mineralized areas within the cave contain barite, calcite, quartz, gypsum, carnotite, a black, botryoidal Ba-Mn oxide that XRD shows to be romanechite, and small (0.1 mm) white sphere-like crystal clusters of dolomite atop the romanechite.
Lechuguilla Cave, in Carlsbad Caverns National Park, New Mexico (Hill, CRF 1986 Annual Report, p. 16-18) has been the subject of extensive recent explorations and discoveries; it is now the deepest, 457 m (1500 feet) and the longest, 34.3 km (21.3 miles) known cave in the park and in the state. A silvery black, slippery, manganese oxide material that is a corroded limestone residue is composed of rancieite, (Ca,Mn+2)Mn+4409•3H20, and todorokite, (Mn+2,Ca,Mg)Mn+4307•H20. Associated brown-colored material contains calcite microcrystals plus nearly amorphous Fe¬Mn oxides and rare grains of an apparent aluminum hydroxide mineral (gibbsite?).

An unusual cave in Precambrian gneiss was discovered in Clear Creek Canyon, west of Golden, Colorado, during blasting for road construction in May 1988 (Modreski and others, 1988, GSA Abstracts with Programs, v. 20, no. 7, p. A65). A 50-m-long x 10-m-wide x 20-m-high open void formed through collapse,
settling, and dissolution of loose rock along a fault zone. The cave contains a spectacular array of carbonate speleothems, including calcite flowstone and draperies, and aragonite stalactites, stalagmites, helictites, and "frostwork" crystal druses. Other minerals identified in the cave include small amounts of hydromagnesite ("moonmilk"); minute blebs of green-luminescent opal as a coating on aragonite crystals; a black Ba¬Mn oxide that contains trace Cu, Co, Mo, W, and Pb and gives an XRD pattern of poorly crystalline romanechite; and a poorly crystalline, Mg-rich, Al-poor, layer silicate mineral (a trioctahedral smectite?) with a platy to spheroidal or botryoidal morphology.

The abundance of aragonite vs calcite speleothems, the relationship of speleothem type to water chemistry and evaporation rate, the correlation of luminescence to trace-element chemistry, and the nature and chemistry of Fe-Mn oxide deposits are some of the topics in mineralogy that may help shed light on the mysteries of caves.